Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Regulatory assets	$ 1,792.4	$ 1,737.3
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	802.9	820.6
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	532.6	542.3
Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	112.1	110.8
EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	107.3	111.6
IPL's DAEC PPA amendment [Member]
Regulatory Assets [Line Items]
Regulatory assets	106.5	0.0
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	23.0	23.6
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	16.9	28.0
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	91.1	100.4
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,351.4	1,279.0
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	770.2	783.1
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	269.3	274.0
IPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	77.5	76.3
IPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	53.6	55.4
IPL [Member] | IPL's DAEC PPA amendment [Member]
Regulatory Assets [Line Items]
Regulatory assets	106.5	0.0
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	23.0	23.6
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	8.0	15.1
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	43.3	51.5
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	441.0	458.3
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	32.7	37.5
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	263.3	268.3
WPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	34.6	34.5
WPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	53.7	56.2
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	8.9	12.9
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 47.8	$ 48.9